In conventional superconductors, very narrow superconducting-fluctuation regions are observed above $T_c$, because strong overlap of Cooper pairs occurs in a coherence volume $4\pi {\xi }^3/3$ with $\xi$ being the coherence length. In the bulk form of iron-chalcogenide superconductor FeSe, it is argued that the system may be located in the crossover region from Bardeen-Cooper-Schrieffer to Bose-Einstein condensation (BEC), where strong superconducting fluctuations are expected. In this respect, we carried out magnetization, specific heat, and Nernst effect measurements on FeSe single crystals in order to investigate the superconducting fluctuation effect near $T_c$. The temperature range of diamagnetization induced by superconducting fluctuations seems very narrow above $T_c$. The temperature-dependent magnetization curves measured at different magnetic fields do not cross at a single point. This is in sharp contrast to the situation in many cuprate superconductors, where such a crossing point has been taken as a clear signature of strong critical fluctuations. The magnetization data can be scaled according to the Ginzburg-Landau fluctuation theory for a quasi-two-dimensional system. However the scaling result cannot be described by the theoretical function of the fluctuation theory due to the limited fluctuation regions. The specific heat jump near $T_c$ is rather sharp without the trace of strong superconducting fluctuations. This is also supported by the Nernst effect measurements which indicate a very narrow region for vortex motion above $T_c$. Associated with a very small value of Ginzburg number and further analyses, we conclude that the superconducting fluctuations are vanishingly weak above $T_c$ in this material. Our results are strongly against the picture of significant phase fluctuations in FeSe single crystals, although the system has a very limited overlap of Cooper pairs in the coherence volume. This dichotomy provides new insights into the superconducting mechanism when the system is with a dilute superfluid density.

A pair of type-II Dirac cones in ${\mathrm{PdTe}}_2$ was recently predicted by theories and confirmed in experiments, making ${\mathrm{PdTe}}_2$ the first material that processes both superconductivity and type-II Dirac fermions. In this paper, we study the evolution of Dirac cones in ${\mathrm{PdTe}}_2$ under hydrostatic pressure by first-principles calculations. Our results show that the pair of type-II Dirac points disappears at 6.1 GPa. Interestingly, a new pair of type-I Dirac points from the same two bands emerges at 4.7 GPa. Due to the distinctive band structures compared with those of ${\mathrm{PtSe}}_2$ and ${\mathrm{PtTe}}_2$, the two types of Dirac points can coexist in ${\mathrm{PdTe}}_2$ under proper pressure (4.7–6.1 GPa). The emergence of type-I Dirac cones and the disappearance of type-II Dirac ones are attributed to an increase/decrease of the energy of the states at the $\mathrm{\Gamma }$ and $A$ points, which have antibonding/bonding characters of the interlayer Te-Te atoms. On the other hand, we find that the superconductivity of ${\mathrm{PdTe}}_2$ slightly decreases with pressure. The pressure-induced types of Dirac cones combined with superconductivity may open a promising way to investigate the complex interactions between Dirac fermions and superconducting quasiparticles.

We propose a refined scheme of deriving an effective low-energy Hamiltonian for materials with strong electronic Coulomb correlations beyond density functional theory (DFT). By tracing out the electronic states away from the target degrees of freedom in a controlled way by a perturbative scheme, we construct an effective Hamiltonian for a restricted low-energy target space incorporating the effects of high-energy degrees of freedom in an effective manner. The resulting effective Hamiltonian can afterwards be solved by accurate many-body solvers. We improve this “multiscale ab initio scheme for correlated electrons” (MACE) primarily in two directions by elaborating and combining two frameworks developed by Hirayama et al. [M. Hirayama, T. Miyake, and M. Imada, Phys. Rev. B 87, 195144 (2013)] and Casula et al. [M. Casula, P. Werner, L. Vaugier, F. Aryasetiawan, T. Miyake, A. J. Millis, and S. Biermann, Phys. Rev. Lett. 109, 126408 (2012)]: (1) Double counting of electronic correlations between the DFT and the low-energy solver is avoided by using the constrained $GW$ scheme; and (2) the frequency dependent interactions emerging from the partial trace summation are successfully separated into a nonlocal part that is treated following ideas by Hirayama et al. and a local part treated nonperturbatively in the spirit of Casula et al. and are incorporated into the renormalization of the low-energy dispersion. The scheme is favorably tested on the example of ${\mathrm{SrVO}}_3$.

A particularly promising pathway to enhance the efficiency of thermoelectric materials lies in the use of resonant states, as suggested by experimentalists and theorists alike. In this paper, we go over the mechanisms used in the literature to explain how resonant levels affect the thermoelectric properties, and we suggest that the effects of hybridization are crucial yet ill understood. In order to get a good grasp of the physical picture and to draw guidelines for thermoelectric enhancement, we use a tight-binding model containing a conduction band hybridized with a flat band. We find that the conductivity is suppressed in a wide energy range near the resonance, but that the Seebeck coefficient can be boosted for strong enough hybridization, thus allowing for a significant increase of the power factor. The Seebeck coefficient can also display a sign change as the Fermi level crosses the resonance. Our results suggest that in order to boost the power factor, the hybridization strength must not be too low, the resonant level must not be too close to the conduction (or valence) band edge, and the Fermi level must be located around, but not inside, the resonant peak.

Here we study how interface and edge perturbations as well as a size effect can be used to manipulate the transport properties in semiconductor heterostructures where a thin film of three-dimensional topological insulator (TI) is sandwiched by normal insulator (NI) slabs. Within the framework of the NI/TI/NI trilayer model based on a continual scheme, we argue that characteristics of electron states in the TI film (energy spectrum, envelope function profile, the Berry curvature, etc.) are controlled by the film thickness and TI/NI interface potential whose variation can lead to the modification of topological properties of the system. Calculating a spin Hall response for the NI/TI/NI trilayer infinite in the interface plane, we find that a series of quantum transitions between topological insulating phase and trivial band insulator phase can be induced by tuning both the film thickness and the interface potential. We draw in detail the corresponding phase diagram of the NI/TI/NI trilayer, which is controlled by change of the sign of either the hybridization gap or the dispersion parameter. To quantify the edge effect, we formulate a model of the half-infinite in the interface plane NI/TI/NI trilayer, which describes evanescent edge states and provides the necessary conditions under which they exist. It is found that the presence of the in-gap edge states is ambiguously determined by the phase of the TI film. Our findings provide a useful guide in choosing the relevant material parameters to facilitate the observation of quantum spin Hall effect in the TI/NI heterostructures.

Two-dimensional (2D) half-metallic materials are essential to develop next-generation spintronic devices. Moreover, electrical controllability and room-temperature magnetism are two important ingredients for applications in spintronics. Here, we report findings of a combination of these properties in 2D honeycomb structures of group-V $\mathrm{N}X$ ($X$=N, P, As, Sb, Bi) binary compounds from first-principles calculations. These novel 2D materials are stable semiconductors with indirect band gaps. Hole doping can induce magnetism due to their Mexican-hat band edges and make them half-metals. Some of these half-metals exhibit inverse spin-polarization direction when changing the doping level, which can be achieved by changing the external voltage gate. Monte Carlo simulations based on the Ising model suggest the Curie temperatures of these half-metals are much higher than room temperature. These outstanding properties make monolayer $\mathrm{N}X$ promising candidates for spintronic applications.

We have observed the Fano-type Faraday rotation spectrum in a one-dimensional magnetophotonic crystal at the Bloch surface wave (BSW) resonance and found that it is a result of the coupling between the $s$-polarized BSW and the $p$-polarized waveguiding modes of the magnetophotonic crystal. The shape of the resonance is determined by the relative position and spectral splitting of the modes.

We analyze the pressure-induced metal-insulator transition in a two-dimensional vertical stack of ${\text{H}}_2$ molecules in (x-y) plane, and show that it represents a striking example of the Mott-Hubbard-type transition. Our combined exact diagonalization approach, formulated and solved in the second quantization formalism, includes also simultaneous ab initio readjustment of the single-particle wave functions, contained in the model microscopic parameters. The system is studied as a function of applied side force (generalized pressure), both in the ${\text{H}}_2$-molecular and $\text{H}$-quasiatomic states. Extended Hubbard model is taken at the start, together with longer-range electron-electron interactions incorporated into the scheme. The stacked molecular plane transforms discontinuously into a (quasi)atomic state under the applied force via a two-step transition: the first between molecular insulating phases and the second from the molecular to the quasiatomic metallic phase. No quasiatomic insulating phase occurs. All the transitions are accompanied by abrupt changes of the bond length and the intermolecular distance (lattice parameter), as well as by discontinuous changes of the principal electronic properties, which are characteristic of the Mott-Hubbard transition here associated with the jumps of the predetermined equilibrium lattice parameter and the effective bond length. The phase transition can be interpreted in terms of the solid hydrogen metallization under pressure exerted by, e.g., the substrate covered with a monomolecular ${\text{H}}_2$ film of the vertically stacked molecules. Both the Mott and Hubbard criteria at the insulator to metal transition are discussed.

Electrical resistivity, magnetization, and specific heat have been measured for single crystals of ${\mathrm{SmPt}}_2{\mathrm{Cd}}_{20}$. It has been found that ${\mathrm{SmPt}}_2{\mathrm{Cd}}_{20}$ exhibits a ferromagnetic (FM) transition at $T_{\mathrm{C}}=0.64$ K, the lowest among cubic compounds. Specific heat divided by temperature increases with decreasing temperature even below $T_{\mathrm{C}}$ and attains 4.5 J  ${\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ at 0.26 K, implying substantial magnetic quantum fluctuations. An analysis of the magnetic entropy suggests the crystalline-electric-field splitting of the Sm $J=5/2$ multiplet with a ${\mathrm{\Gamma }}_7$ doublet ground state and a ${\mathrm{\Gamma }}_8$ quartet excited state (the excitation energy of $\sim 30\mathrm{K}$). The electrical resistivity shows a power-law behavior with $T^{0.74}$ below 2 K without showing any noticeable anomaly at $T_{\mathrm{C}}$. ${\mathrm{SmPt}}_2{\mathrm{Cd}}_{20}$ is regarded as a rare cubic system that is located in the vicinity of a FM quantum critical point.

The conventional tensor-network states employ real-space product states as reference wave functions. Here, we propose a many-variable variational Monte Carlo (mVMC) method combined with tensor networks by taking advantages of both to study fermionic models. The variational wave function is composed of a pair product wave function operated by real-space correlation factors and tensor networks. Moreover, we can apply quantum number projections, such as spin, momentum, and lattice symmetry projections, to recover the symmetry of the wave function to further improve the accuracy. We benchmark our method for one- and two-dimensional Hubbard models, which show significant improvement over the results obtained individually either by mVMC or by tensor network. We have applied the present method to a hole-doped Hubbard model on the square lattice, which indicates the stripe charge/spin order coexisting with a weak $d$-wave superconducting order in the ground state for the doping concentration of less than 0.3, where the stripe oscillation period gets longer with increasing hole concentration. The charge homogeneous and highly superconducting state also exists as a metastable excited state for the doping concentration less than 0.25.

Recent experiments performed on cuprates and alkali-doped fullerides have demonstrated that key signatures of superconductivity can be induced above the equilibrium critical temperature by optical modulation. These observations in disparate physical systems may indicate a general underlying mechanism. Multiple theories have been proposed, but these either consider specific features, such as competing instabilities, or focus on conventional BCS-type superconductivity. Here we show that periodic driving can enhance electron pairing in strongly correlated systems. Focusing on the strongly repulsive limit of the doped Hubbard model, we investigate in-gap, spatially inhomogeneous, on-site modulations. We demonstrate that such modulations substantially reduce electronic hopping, while simultaneously sustaining superexchange interactions and pair hopping via driving-induced virtual charge excitations. We calculate real-time dynamics for the one-dimensional case, starting from zero- and finite-temperature initial states, and we show that enhanced singlet-pair correlations emerge quickly and robustly in the out-of-equilibrium many-body state. Our results reveal a fundamental pairing mechanism that might underpin optically induced superconductivity in some strongly correlated quantum materials.

The tensile-strained $\mathrm{Si}\left(111\right)$ layers grown on top of $\mathrm{Ge}\left(111\right)$ substrates are studied by combining scanning tunneling microscopy, low-energy electron diffraction, and first-principles calculations. It is shown that the layers exhibit $c(2\times 4)$ domains, which are separated by domain walls along $〈\overline{1}10〉$ directions. A model structure for the $c(2\times 4)$ domains is proposed, which shows low formation energy and good agreement with the experimental data. The results of our calculations suggest that Ge atoms are likely to replace Si atoms with dangling bonds on the surface (rest-atoms and adatoms), thus significantly lowering the surface energy and inducing the formation of domain walls. The experiments and calculations demonstrate that when surface strain changes from compressive to tensile, the (111) reconstruction converts from dimer-adatom-stacking fault-based to adatom-based structures.

The spontaneous orientational order of some metal islands on Cu(001) along $\langle 100\rangle$ has long eluded its explanation, which is now revealed by the study with the kinetic Monte Carlo simulation. The orientational order originates from the relative stability of the diagonal orientation of the neighboring islands against their coalescence. That stability results from the square symmetry of the islands, since the diagonally meeting islands have the minimum contact area mediating their merge. Moreover, we well reproduce and elucidate the orientational order propagating into the following several tens of monolayers. The orientational order of the islands is driven simply by the symmetry of the surface unit cell and is thus generic in the epitaxial growth, making pervasive effects on the physical properties of the epitaxial films.

The in situ x-ray pair-distribution function (PDF) characterization technique has been used to study the behavior of ${\left({\mathrm{K}}_x\mathrm{N}{\mathrm{a}}_{1-x}\right)}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_3$, as a function of electric field. As opposed to conventional x-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the angstrom to nanometer scale. Several field-dependent ordering mechanisms can be observed in $x=0.15$, 0.18 and at the morphotropic phase boundary composition $x=0.20$. X-ray total scattering shows suppression of diffuse scattering with increasing electric-field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks in the 3–4-Å range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric-field vector. A transition from $\langle 110\rangle$ to $\langle 112\rangle$-type off-center displacements of Bi relative to the neighboring Ti atoms is observable with increasing $x$. Analysis of PDF peak shift with electric field shows the effects of Bi-Ti redistribution and onset of piezoelectric lattice strain. The combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.

We report $3\omega$ thermal conductivity measurements of amorphous and nanocrystalline silicon thin films from 85 to 300 K prepared by hot-wire chemical-vapor deposition, where the crystallinity of the films is controlled by the hydrogen dilution during growth. The thermal conductivity of the amorphous silicon film is in agreement with several previous reports of amorphous silicon prepared by a variety of deposition techniques. The thermal conductivity of the as-grown nanocrystalline silicon film is 70% higher and increases 35% more after an anneal at $600{}^{\circ }\mathrm{C}$. They all have similarly weak temperature dependence. Structural analysis shows that the as-grown nanocrystalline silicon is approximately $60\%$ crystalline, nanograins and grain boundaries included. The nanograins, averaging 9.1 nm in diameter in the as-grown film, are embedded in an amorphous matrix. The grain size increases to 9.7 nm upon annealing, accompanied by the disappearance of the amorphous phase. We extend the models of grain boundary scattering of phonons with two different non-Debye dispersion relations to explain our result of nanocrystalline silicon, confirming the strong grain size dependence of heat transport for nanocrystalline materials. However, the similarity in thermal conductivity between amorphous and nanocrystalline silicon suggests the heat transport mechanisms in both structures may not be as dissimilar as we currently understand.

We report a comparison of the $1/T_1$ spin-lattice relaxation rates for ${}^9\mathrm{Li}$ and ${}^8\mathrm{Li}$ in Pt and ${\mathrm{SrTiO}}_3$, in order to differentiate between magnetic and electric quadrupolar relaxation mechanisms. In Pt, the ratio of the $1/T_1$ spin relaxation rates $R_{\text{Pt}}$ was found to be 6.82(29), which is close to but less than the theoretical limit of $\sim 7.68$ for pure magnetic relaxation. In ${\mathrm{SrTiO}}_3$ this ratio was found to be 2.7(3), which is close to but larger than the theoretical limit of $\sim 2.14$ expected for pure electric quadrupolar relaxation. These results bring insight into the nature of the fluctuations in the local environment of implanted ${}^8\mathrm{Li}$ observed by $\beta$-NMR.

We study hard core bosons on a two-leg ladder lattice under the orbital effect of a uniform magnetic field. At densities which are incommensurate with flux, the ground state is a Meissner state, or a vortex state, depending on the strength of the flux. When the density is commensurate with the flux, analytical arguments predict the possibility to stabilize a ground state of central charge $c=1$, which is a precursor of the two-dimensional Laughlin state at $\nu =1/2$. This differs from the coupled wire construction of the Laughlin state in that there exists a nonzero backscattering term in the edge Hamiltonian. By using a combination of bosonization and density matrix renormalization group (DMRG) calculations, we construct a phase diagram versus density and flux from local observables and central charge. We delimit the region where the finite-size ground state displays signatures compatible with this precursor to the Laughlin state. We show how bipartite charge fluctuations allow access to the Luttinger parameter for the edge Luttinger liquid corresponding to the precursor Laughlin state. The properties studied with local observables are confirmed by the long distance behavior of correlation functions. Our findings are consistent with an exact-diagonalization calculation of the many body ground state transverse conductivity in a thin torus geometry for parameters corresponding to the precursor Laughlin state. The model considered is simple enough such that the precursor to the Laughlin state could be realized in current ultracold atom, Josephson junction array, and quantum circuit experiments.

Electronic, lattice, and spin interactions at the interfaces between crystalline complex transition-metal oxides can give rise to a wide range of functional electronic and magnetic phenomena not found in bulk. At heterointerfaces, these interactions may be enhanced by combining oxides where the polarity changes at the interface. The physical structure between nonpolar $\mathrm{SrTi}{\mathrm{O}}_3$ and polar $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ ($x=0.2$) is investigated using high-resolution synchrotron x-ray diffraction to directly determine the role of structural distortions in compensating the polar discontinuity. At both the oxide-oxide interface and vacuum-oxide interfaces, the lattice is found to expand and rumple along the growth direction. The $\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ interface also exhibits intermixing of La and Sr over a few unit cells. The results thus demonstrate that polar distortions and ionic intermixing coexist and both pathways play a significant role at interfaces with polar discontinuities.

In magnetic materials, both electrons and magnons are capable of carrying angular momentum currents. An external electric field can efficiently drive a charge and spin current of electrons, but it is unable to directly produce a chargeless magnon current. The generation of the magnon current is conventionally achieved via thermal gradients or the electron spin injection from interfaces. Here, we investigate the magnon current induced by the momentum and angular momentum transfer from conduction electrons in magnetic layered systems. By using the generic exchange interaction between electrons and magnons, we derive the coupled diffusion equations for electron spins and magnons and we find (a) the ratio between the magnon current and the electric charge current is substantial at room temperature for conventional conducting ferromagnets, (b) the spin diffusion length of electrons is significantly modified by the presence of the nonequilibrium magnon density, and (c) the giant magnetoresistance of the magnetic multilayers for the current perpendicular to the plane of layers is reduced compared to the prior theory without taking into account the magnon current.